For many years the manufacture of cement from cement raw meal has taken place in plants of the type having:
(a) a preheater with an inlet and outlet for heating gas and an inlet and an outlet for pulverous cement raw material;
(b) a suspension calciner with inlets for fuel, combustion air and preheated raw material, an outlet for combustion gas connected to the heating gas inlet of the preheater and an outlet for calcined material communicating with a material inlet of;
(c) a sintering furnace with inlet for fuel and combustion air, an outlet for combustion gas communicating directly or indirectly with the heating gas inlet of the preheater and an outlet for the sintered product connected to a material inlet of;
(d) an air cooler for cooling the sintered product with an air outlet connected to both the air inlet of the sintering furnace and the air inlet of the suspension calciner.
Plants of this kind are known from U.S. Pat. No. 3,203,681 and British Patent Specification No. 1,434,091 according to which a short rotary kiln is used as sintering furnace.
The characteristic feature of this kind of plant is that the heat treatment of the cement raw material necessary for the manufacture of cement clinker takes place in three separate zones of the plant corresponding to the three heat treatment phases: preheating substantially to calcining temperature; calcining; and final heat treatment comprising heating to sintering temperature and sintering.
By this process improved heat economy and improved control of the phases of the process are achieved as compared with earlier plants in which the entire heat treatment took place in a long rotary kiln.
The rotary kiln is however not an ideal apparatus, the essential drawback being the relatively inefficient heat transfer during the process of heating the material from calcining to sintering temperature. Because of this the machinery dimensions are necessarily large, resulting in high initial apparatus cost, a substantial heat loss, and a considerable thermal inertia leading to long starting up periods as well as control problems.
Another disadvantage of the rotary kiln as a sintering reactor is the rather limited number of variable process parameters which are available. As a result it is difficult at the same time to optimise both the heating process and the crucial sintering process where the formation of the clinker minerals takes place.
Over the years numerous proposals have been made for sintering furnaces which do not present the inherent disadvantages of the rotary kiln. See, for example, U.S. Pat. Nos. 2,776,132 and 3,013,786 (sintering in fluid bed); and U.S. Pat. No. 2,489,211, British Patent Specification 959,446 and U.S. Pat. No. 3,603,568 (sintering in suspended state). However, essentially due to the sticky nature of cement raw material at sintering temperature, none of these solutions have functioned quite satisfactorily, which accounts for the fact that up to now the final heat treatment is still in practice carried out in a rotary kiln.
I have invented an apparatus of the kind described in which the final heat treatment can be performed in small compact machinery under easy control, while minimizing the heat loss and problems caused by the sticky nature of the sintering material.